A phylum of pseudocoelomate invertebrates comprising the roundworms. They are characterized by a smooth narrow cylindrical unsegmented body tapered at both ends. They shed their tough outer cuticle four times during life to allow growth. The microscopic free-living forms are found in all parts of the world, where they play an important role in the breakdown and recycling of organic matter. The many parasitic nematodes of plants and animals may be much larger; they include the filaria (Wuchereria) and Guinea worm (Dracunculus), which cause serious diseases in humans. Some are serious crop pests, such as eelworms, which attack plant roots. Nematodes belong to the clade Ecdysozoa. See also caenorhabditis elegans.

https://nematologists.org/ Website of the Society of Nematologists; Education Committee link accesses many images of nematodes